Consumers so far this holiday shopping season have spent more than $19.57 billion with online retailers, up 15.1% from the more than $17.00 billion spent at the same point last year, comScore Inc. said today in its latest e-commerce spending estimate. The web measurement firm’s figures account for spending between Nov. 1 and Dec. 4.

This year’s spending surged in large part because of three billion-dollar days for e-commerce: Cyber Monday, which stands as the most lucrative day for U.S. e-commerce yet, along with Nov. 29 and Nov. 30. The only other billion-dollar day in the history of e-commerce was last year’s Cyber Monday, that is, the Monday after Thanksgiving.

For the week ending Dec. 4, e-commerce spending topped $5.80 billion, up 15% from the same week last year, comScore adds. Now, though, comes the potentially soft part of the holiday shopping season for online retailers, says Gian Fulgoni, comScore’s chairman.

“We are now entering the portion of the season when online retailers typically reduce their promotional activity as they attempt to restore their margins, which can sometimes contribute to temporary softness in spending growth rates,” he says. “However, this will likely be immediately followed with an acceleration in the spending rate as we approach the middle of December, which is sure to bring us several more billion dollar online spending days.”

Better-off consumers account for the growth in online holiday spending, with households that earn between $50,000 and $99,000 annually increasing their online spending the most, comScore says. Those mid-income households this holiday season are spending $175 per buyer, up nearly 20% from $146 at the same point last year. Households that earn more than $100,000 per year are spending $186 per buyer, up 16% from last year, while households that earn less than $50,000 per year are spending $134 per buyer, down 1% from last year.

 “We have seen some indications that mid-income Americans might be experiencing ‘austerity fatigue,’” Fulgoni says. “After spending the last couple of years increasing their personal savings in an effort to get their finances in order, some appear to have decided they are done pinching pennies and are increasing their spending by taking advantage of the many promotions and price discounts that online retailers have been offering in the first half of the holiday shopping season. The appeal of lower prices that can be found on the Internet is also undoubtedly a driver of the acceleration of the shift from in-store buying to e-commerce that we are witnessing among all income segments this season.”

The number of households buying online has grown only modestly, by 3%, this holiday season, to 119.7 million households from 116.2 million at this point last year. That’s a departure from past years when it was growth in the number of online buyers that drove growth; this year it is the 12% average increase in spend per buyer, comScore says.